Rotational band based on πf7/2 orbital in 55Mn

The excited states of odd-A nucleus ⁵⁵Mn have been investigated using α-induced fusion evaporation reaction to explore the shape driving effect of proton f_7/2 orbital below Z = 28 shell gap. A deformed band structure, based on $\pi f_{7/2}$ configuration has been identified at lower excitation with firm spin-parity assignments of the levels. Band crossing has been identified for the first time in this nucleus through the observation of back bending in one of the signature partners. The properties of this band has been compared with the similar bands in the neighboring isotopes, ^57,59Mn. The Total Routhian Surface calculations predict prolate shape for all the three Mn isotopes below the band crossing. However, a change in shape is predicted by these calculations at higher frequencies beyond the particle alignment. An unnatural parity state has also been identified in ⁵⁵Mn which has been found to decay by E3 and E1 transitions. This observation indicates, for the first time, the presence of octupole correlation in this nucleus. It has been discussed in the light of the octupole correlation observed in its isobar ⁵⁵V and other nearby odd-odd nuclei.